The present invention relates to a tilt angle measuring device such as a bubble tube, which is prepared by sealing liquid and air bubble into a glass container so that the air bubble is moved depending on a tilt angle of the glass container.
A bubble tube is used to detect tilting of a device or equipment. This bubble tube comprises a glass container where liquid and air bubble are sealed in. When a system or equipment such as survey instrument is installed, which requires horizontality, a tilt angle measuring device with the bubble tube is used. The movement of air bubble in the bubble tube is photoelectrically detected by a photoelectric bubble tube. Now, a description will be given on a conventional type tilt angle measuring device equipped with such photoelectric bubble tube referring to FIG. 8.
A photoelectric bubble tube 1 comprises a light source 2, photodetection elements 3 and 4, and a bubble tube 5. Tilt angle is detected according to a signal from the photoelectric bubble tube 1.
The light source 2 is positioned under the bubble tube 5, and the optical axis of the light source 2 runs perpendicularly to the axis of the bubble tube 5. A pair of the photodetection elements 3 and 4 is placed at positions opposite to the light source 2 in such manner that the bubble tube 5 is interposed between the photodetection elements. The photodetection elements are separated by a predetermined distance and arranged at positions symmetrical to the optical axis of the light source 2.
A detection light beam 7 emitted from the light source 2 passes through the bubble tube 5 and reaches the photodetection elements 3 and 4. Photodetection currents corresponding to the quantity of the light received by the photodetection elements 3 and 4 are outputted.
The detection light beam 7 emitted from the light source 2 passes through a part of the bubble tube 5 where there is only liquid. In a portion of the bubble tube where there is air bubble, the detection light beam 7 is reflected by boundary surface between the air bubble 12 and the liquid 11. As a result, the detection light beam 7 entering the photodetection elements 3 and 4 is cut off. Therefore, the quantity of the detection light beam 7 entering the photodetection elements 3 and 4 varies according to the position of the air bubble 12. The position of the air bubble 12 moves according to the tilting. Thus, by finding difference of the quantity of the photodetection amount between the photodetection elements 3 and 4, the tilt angle can be detected.
As the liquid to be sealed in the bubble tube of the tilt angle measuring device, hydrocarbons of an alcohol type or an ether type or a mixed solution of these compounds has been widely used in the past. For example, a mixed solution of ethanol and diethyl ether, or a mixed solution of n(normal)-hexane and n(normal)-heptane have been used.
FIG. 9 shows the results of an experiment to determine detection accuracy of the bubble tube 5 where a conventional mixed solution of hydrocarbon type (in this case, n-hexane and n-heptane) is used. FIG. 9 shows an example of the results of measurement on the relation between a tilt angle and a sensor output in the range of xc2x14xe2x80x2 when temperature of the sealed liquid is sequentially changed in a cycle of 25xc2x0 C.xe2x86x92xe2x88x9220xc2x0 C.xe2x86x9250xc2x0 C.xe2x86x9225xc2x0 C.
In this example, the reproducibility in the range of xc2x14xe2x80x2 at 25xc2x0 C. is 10xe2x80x3 at maximum, temperature shift of zero point (origin) in the range of xe2x88x9220xc2x0 C. to 50xc2x0 C. is about 0.12xe2x80x3/xc2x0 C., temperature drift of sensitivity in the range of xc2x13xe2x80x2 is about 12%. Linearity (operating range) is obtained in the range of xc2x13xe2x80x2.
However, in the conventional type bubble tube 5 containing hydrocarbon mixed solution (n-hexane and n-heptane), the reproducibility, the temperature shift of zero point, the temperature drift and the linearity as given above are not sufficiently efficient, and this is attributed to the liquid sealed in the bubble tube. More concretely, the causes are as follows:
That is, the mixed solution used as the liquid sealed in the bubble tube has a high thermal expansion coefficient. For this reason, the size of air bubble sealed in the bubble tube changes widely according to the environmental temperature.
The container where the mixed solution is sealed is made of glass, but the wettability of the mixed solution to glass is low. As a result, air bubble does not move smoothly.
Further, the mixed solution has low thermal conductivity and high specific heat. Accordingly, it does not follow well after environmental temperature change, and this adversely affects the performance characteristics of the bubble tube.
It is an object of the present invention to provide a tilt angle measuring device, in which a mixed solution having a thermal expansion coefficient of about 1/100 or less of that of the conventionally used solution, having good wettability to glass, and having high thermal conductivity, is sealed in a glass container together with air bubble.
To attain the above object, the tilt angle measuring device according to the present invention comprises a glass container with a solution and air bubble sealed therein for measuring a tilt angle by detecting displacement of the air bubble, wherein the solution contains N-methyl-formamide and a solvent. Further, the present invention provides the tilt angle measuring device as described above, wherein the solvent is a solvent selected from at least one of nitrogen compounds such as acetonitrile, alcohols such as methanol, ethanol, etc., phenols, ethers, acetals, ketones, esters, fatty acids, acid anhydrides, sulfur compounds, compounds having two or more functional groups, or inorganic solvents. Also, the present invention provides the tilt angle measuring device as described above, wherein mixing ratio of N-methylformamide and the solvent is 3:7 in volume ratio.
N-methylformamide as described above has a thermal expansion coefficient of about 1/100 or less of that of a hydrocarbon solution such as methanol, ethanol, n-hexane, n-heptane, etc. as used in the past, has higher thermal conductivity, and better wettability to glass compared with the conventionally used solution.
When a solvent selected from at least one of the following compounds is mixed as a diluent, viscosity of N-methylformamide is decreased: nitrogen compounds such as acetonitrile, alcohols such as methanol, ethanol, etc., phenols, ethers, acetals, ketones, esters, fatty acids, acid anhydrides, sulfur compounds, compounds having two or more functional groups, or inorganic solvents.
When N-methylformamide and the above solvent are mixed at a mixing ratio of 3:7, bubble length temperature change (%) is very low and mobility of air bubble is more satisfactory.